1. Field of the Invention
The invention relates to a discharge lamp of the short arc type. The invention relates especially to a discharge lamp of the short arc type which is used as the exposure light source for producing semiconductor components, liquid crystal display (LCD) elements, printed circuit boards and the like.
2. Description of Related Art
In the manufacture of semiconductor components, liquid crystal display elements, and the like, so-called photolithography is used in which a mask pattern is irradiated with UV radiation and a workpiece is exposed by this UV radiation which has passed through the mask pattern. In this photolithography, the light source is a discharge lamp of the short arc type which is subjected to vertical operation in which the cathode is pointed upward.
The arc tube of this discharge lamp is filled with emission substances such as a rare gas, mercury and the like, and mainly UV radiation is emitted. However, during lamp operation, the material which forms the anode and cathode, such as tungsten, vaporizes. This vaporized tungsten flows by the convection present in the arc tube, is usually deposited on the inner wall in the upper area of the arc tube, causes blackening and reduces the amount of UV radiation transmission.
In this case of using this discharge lamp for an exposure device and the like, radiant light is used in only one area at a predetermined angle with reference to the arc center. This area has an angle of, for example, 40.degree. to the top and 35.degree. to the bottom with reference to the arc center.
To eliminate this disadvantage of reduction of the amount of UV radiation transmission as a result of tungsten vaporization, an inner lead which supports the cathode is used which is longer than the inner lead which supports the anode. In this way, the cathode tip is positioned under the center of the arc tube, and thus, a larger space for the upper region of the arc tube is obtained. As a result, the upper space of the arc tube can be used outside the above described angular range.
But, in the case in which, by increasing the distance between the back end of the cathode and the hermetically sealed portion, a larger space in the upper part of the arc tube is used, undesirable turbulence of the gas occurs in the vicinity of the inner lead which supports the cathode during gas convection in the arc tube. The reason for this is that the outside diameter of the inner lead is much smaller than the outside diameter of the cathode and that the inner lead which supports the cathode is relatively long in order to position the cathode tip under the middle of the arc tube, as was described above. This turbulence of the convection gas causes turbulence during convection of all the gas in the arc tube. The arc fluctuates due to this perturbed convection and as a result the radiant light intensity becomes unstable.
The reason that the outside diameter of the inner lead is smaller than the outside diameter of the cathode is the following:
If the outside diameter of the inner lead is large, production of the hermetically sealed portion is difficult. PA1 It is enough if the inner lead can support the cathode. It is therefore effective to reduce the thickness of the inner lead if no problem arises during support, and thus to reduce production costs.
Furthermore, the reason why the outside diameter of the cathode is smaller than the outside diameter of the inner lead is that an increase of cathode volume can increase thermal conductivity and that thus a temperature increase of the cathode can be prevented.
Especially, recently there has been a trend toward detailing the mask pattern used for photolithography. Therefore, in the case of fluctuation of the arc and unstable radiant light intensity, there is the disadvantage that exposure with high image resolution is no longer enabled.
Recently, with respect to the increase of production efficiency, there has been a demand for a discharge lamp of the short arc type with a large amount of UV radiation. Therefore, a discharge lamp of the short arc type with high input power, for example, greater than or equal to 2 kW, is used.
Accordingly, in the case of this discharge lamp of the short arc type with high input power, enlargement of the arc tube and thus of the volume of the discharge space occurs. The above described turbulence in the case of convection, therefore, becomes apparent. As a result, the disadvantage exists that the arc fluctuation and instability of light intensity continue to occur.